RATES AND SCALES OF SEDIMENT PRODUCTION IN THE RIO PUERCO BASIN, NEW MEXICO

The rates at which sediment is generated on hillslopes by weathering (sediment production) is highly variable, poorly known, and difficult to predict. We show, using in-situ-produced 10Be measured in alluvial sediment, that rates of sediment yield and sediment production vary over significantly different temporal and spatial scale in the Rio Puerco Basin of northern New Mexico. Suspended sediment concentrations in the Rio Puerco, the largest tributary of the Rio Grande, are among the highest in the world, up to 600g/l. The alluvial drainage network is deeply incised, the result of an arroyo-cutting episode that began in the late 1800s. Large volumes of unconsolidated sediment are delivered directly to the channel during downcutting and subsequent channel widening. As headcuts develop, soil piping progresses headward and arroyo walls collapse. Radiocarbon dating and the lack of identifiable paleosols, demonstrate that the cutting and filling of Rio Puerco arroyos have occurred repeatedly during the Holocene. Thus, it appears that average sediment storage times in the valley bottom are short, on the order of no more than a few thousand years. To determine the rate at which sediment is generated by rock weathering and to test the relationship between 10Be activity and drainage basin area we collected sand-sized alluvial sediment from 37 sites in the channel and on aggraded alluvium within the boundaries of the 20th century arroyos. Drainage basin areas above the sample sites ranged from 170 km2 to 16,000 km2. We separated quartz from the samples and isolated 10Be using standard techniques4. We use an interpretive model that relates nuclide activity in fluvial sediment to the rate at which drainage basins are eroding and producing sediment. Individual basin-scale rates for sediment generation range from 2.1*104 kg km2 y-1 to 1.0 *106 kg km2 y-1, the equivalent of rock erosion at 8 to 400 m My-1 (_=2600 kg m3). Basin to basin variance is high for smaller basins (<2000 km2) but dampens with increasing basin area, demonstrating the importance and efficiency of sediment mixing during fluvial transport.